Nickel complexes, oxygen reactivity

We have also observed that the sensitivity of aryl-methyl-nickel compounds la and b to oxygen is greatly enhanced by the addition of methylllthium. Under these conditions, the presence of la and b as nickelate complexes in is indicated by isotopic excliange studies. These anionic nT el complexes should be even better donors than their neutral counterparts 1,(26) and they are thus expected also to show enhanced reactivity to aryl bromides in those interactions proceeding by electron transfer. Reductive eliminations similar to those presented for 1 can be formulated as ... 

Nickel complexes act as catalysts for the oxidation of ditertiary diphosphines. Palladium(o) complexes catalyse the autoxidation of hydrocarbons. The reactivity is sensitive to the nature of the arylphosphine ligands. The mechanism involves intermediate formation of a palladium-molecular oxygen complex. ... 

Since cyclooctadiene has no suitable low-lying unoccupied orbitals some of the 3d electrons of nickel are expected to have a relatively high antibonding character. It is therefore not surprising that the nickel complex is extremely reactive, air-sensitive, and very unstable in solutions even in the absence of oxygen. Carbon monoxide at room temperature completely displaces the cyclooctadiene molecules and yields nickel carbonyl (99). Acrylonitrile reacts with (LII) under similarly mild conditions, forming bis(acrylo-nitrile)-nickel 101), while duroquinone, well below room temperature, affords cyclooctadiene-duroquinone-nickel 101). These reactions uniquely demonstrate the close interrelationship between all complexes of zero-valent nickel. 

The impressive sulfur-based reactivity of square planar nickel complexes containing tetradentate N2S2 ligands has been known for many years. Interest has recently resurfaced because of the discovery of similar donor sites in metalloproteins that bind nickel, iron, and cobalt.The iV,iV -bis(mercaptoethyl)-l,5-diazacyclooc-tane ligand H2(BME-D ACO) and its nickel complex have been particularly useful in establishing the scope of S-based reactivity with electrophiles as displayed in the reaction summary shown in Scheme 1." The fundamental features of this reactivity include templated macrocycle production, S-oxygenation as contrasted to oxidation, Lewis acid/base adduct formation, metal-ion capture, and the synthesis of heterodi- and polymetallic complexes. ... 

In Baeyer-Villiger oxidations catalyzed by (achiral) copper or nickel salts substituted cyclohexanones like 4 (Eq. 3) had been shown to be reactive substrates for the conversion to the corresponding (racemic) lactones in the presence of aldehyde and molecular oxygen [22]. The next step was to develop a chiral catalyst in order to make this reaction proceed in an enantioselective manner, giving optically active oxepanones. Various copper complexes were screened in a search for asymmetric induction in the aerobic Baeyer-Villiger oxidation. The most active and selective catalyst that was eventually found was the copper complex... 

Where a reactive lower oxidation state results, a key concern is the necessary protection of the reduced complex from air or other potential oxidants, as they are often readily reoxidized. Usually, this requires their handling in special apparatus such as inert-atmosphere boxes or sealed glassware in the absence of oxygen. Where active metal reducing agents (such as potassium) are employed, special care with choice of solvent is also necessary. The nickel reduction reaction (6.33) can be performed in liquid ammonia as solvent, since the strongly-bound cyanide ions are not substituted by this potential ligand. 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel-oxo complexes (e.g., LNi -0, LNi=0, LNi -0-I-Ph, or LNi -0-Ni L) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

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